The sandwich technique is a specific strategy in restorative dentistry used for fillings. In both open and closed sandwich techniques, the different materials of the composite resin is layered or “stacked” onto the tooth, similar to building the layers of a sandwich. This is done instead of mixing the materials of the resin together before filling the cavity. An open sandwich refers to when the filling is located on one of the sides of the tooth and comes into contact with the oral cavity. A closed sandwich refers to a filling in the center of the tooth that does not come into contact with the oral cavity. Many dental professionals who work in restorative dentistry feel that the sandwich technique provides a stronger filling, because the glass ionomer cement that is layered on first bonds to the tooth structure below and the composite to follow, offering a better seal and increasing filling retention.